use Encode 'decode';

...

my $string = decode('UTF-8', $input_string);

# Keep only what we want:
$string =~ m!([a-zA-Z0-9]+)!
    or warn "Invalid/empty username in '$string'";
my $real_user = $1;

# Remove stuff we don't want, especially the writing direction isolates:
$string =~ s!\x{2066}|\x{2069}!!g;